The Origin of Vertebrates

by Walter Holbrook Gaskell

These sense-organs are identical in structure with those in the skin, and, as Anderson has shown, the nerves of these organs medullate at the same time as those in the skin, and both obtain their medullary sheaths earlier than any other nerves, whether afferent or efferent. However difficult it may be to explain this fact, only one conclusion seems to me possible—these Pacinian bodies, like the skin Pacinians, originate from a nest of surface epithelial cells, a conclusion which is extremely probable on my theory of the origin of vertebrates, but not, as far as I can see, on any other.

At the present moment the weight of evidence is, to my mind, in favour of the lining endothelium of the cœlomic cavities being composed of free cells, unconnected with the nervous system rather than the reverse, but I must confess that the question is undecided. If it be true that the cœlomic lining is partly enterocœlic and partly gonocœlic, as Lankester teaches, then it would be natural that its cells should be in connection with the nervous system, to some extent at all events. This view is, however, based on very slender foundations. If the mesothelium is composed of cells capable of becoming free, it cannot give rise to the skeletal muscles, and it cannot therefore be right to speak of the skeletal muscles as derived from the lining cells of a part of the primary cœlom. The phylogenetic history of the musculature of the different animals points strongly to its intimate connection with and derivation from surface epithelial cells rather than from cœlomic mesothelial cells. Thus in the cœlenterates, as seen in Hydra, the muscular layer arises directly from a modification of the surface epithelial cells; and right up to the annelids, even to the highest form in the Polychæta, we still see it stated that the musculature, both circular and longitudinal, arises from the ectoderm. In the Oligochæta and Hirudinea, according to Bergh, there are five rows of teloblasts on each side, of which four are ectodermic and give rise to the nerve-ganglia and the circular muscles, while one is mesoblastic and forms the nephridial organs and the longitudinal muscles. (The latter statement is, according to Bergh, well known, and is not particularly shown by him. These longitudinal muscle-bands always lie close against the nervous system at their first formation, and may well have been derived in connection with it.)

It is apparently only in the Vertebrata that the lining cells of the cœlomic cavity are definitely stated to give origin to the body-musculature, and taking into account on the one hand the evidence of Graham Kerr as to the intimate connection between nerve-cell and muscle-cell from the very beginning, and on the other the manner in which all the skeletal muscles of the adult are lined with a lymphatic endothelium, I am strongly inclined to believe that at the closing up of the myocœle, when the myomere separates from the mesomere, the lining cells remain scattered in among the forming muscle-cells and form the ultimate lymphatic tissue of the muscles. If this is really so, then the evidence in favour of the mesothelium being composed of free cells not connected with the nervous system would be much strengthened, for, on the one hand, an intimate relation exists between the connective tissue cells and the endothelium of the roots of the lymphatic vessels, a relation which, according to Virchow, has rendered it impossible to draw any sharp line of distinction between the two; and, on the other, the lymphatic endothelium merges into the lining cells of the great serous cavities of the body.

It is impossible to conceive of an animal possessing a nervous system which is not in connection with sensory and muscular tissues; an isolated nerve-cell is a meaningless possession; but it is equally natural to conceive of a germ-cell being isolated, capable of living an independent existence. Such a difference between the two kinds of tissues must have existed from the very commencement of the Metazoa, so that we must, it seems to me, imagine that in the formation of the Metazoa from the Protozoa the whole of the body of the latter did not break up into a mass of separate gonads, each capable of becoming a free-living protozoan similar to its parent, but that a portion proliferated into a multinucleated syncytium while the remainder formed the free-living gonads. This multinucleated syncytium, or host, as it might be called, would still continue to exist for the purpose of carrying further afield the immortal gonads, which need no longer be all shed at one time.

In such an animal as Volvox globator we have an indication of the very kind of animal postulated as connecting the single-celled Protozoa and the multi-cellular Metazoa, for it consists of a many-celled case which forms a hollow sphere, each of the cells being provided with flagella for the purpose of locomotion of the sphere, except a certain number which are not flagellated; the latter leave the case to swim freely in the fluid contained within the sphere, and forming spermaries and ovaries, conjugate, maturate, and then are set free by the rupture of the encircling locomotor host.

This conception of the predecessors of the Metazoa being composed of a mortal host, holding within itself the immortal sexual products, leads naturally to the idea of the separate development of the host from that of the germ-cells ab initio, so that the study of the development of the Metazoa means the study of two separate constituents of the metazoan individual—on the one hand, the elaboration of the elements forming the syncytial host, on the other, of those derived from the free-living independent germ-cells. The elaboration of the host means the differentiation of the protoplasm into epithelial, muscular, and nervous elements, by means of which the gonads were carried further afield and their nourishment as well as that of the host ensured.

The rôle of the nervous system as the middleman between internal and external muscular and epithelial surfaces was, I imagine, initiated from the very earliest time. The further evolution of the host consisted in a greater and greater differentiation and elaboration of this neuro-epithelial syncytium, with the result of a steadily increasing concentration and departmental centralization of the main factor of the syncytium; in other words, it led to the origin and elaboration of a central nervous system. In the interstices of this syncytium the gonads were placed, and at first, doubtless, the life of the host ended when all the germ-cells had been set free. 'Reproduce and die' was, I imagine, the law of the Metazoa at its earliest origin, and throughout the ages, during all the changes of evolution, the reminiscence of such law still manifests itself even up to the highest forms as yet reached. With the differentiation of the syncytial host there came also differentiation of the free-living gonads, so that only some of them attained to the perfection of independent existence, capable of continuing the species; while others became subordinate to the first and provided them with pabulum, manufacturing within themselves yolk-spherules, and thus in the shape of yolk-cells ministered to the developing egg-cell. Thus arose a germinal epithelium of which only a few of the elements passed out of the host as perfect individuals, the remainder being utilized for the nutrition of these few. Such yolk-cells of the germinal epithelium would still, however, retain their character as free cells totally independent of the syncytial host, and, situated as they were between the internal and external epithelium, capable of amœboid movement, would naturally have their phagocytic action utilized either as yolk-cells for the providing of pabulum to the egg-cell, or as excretory cells for the removal and rendering harmless of deleterious products of all kinds. Thus the free cells of the body would become differentiated into the three classes of germ-cells, yolk-cells, and excretory cells.

Further, the mass of gonads, which originally occupied so large a space within the interior of the host, necessarily, as the tissues of the host differentiated more and more, took up less and less space in proportion to the whole bulk of the host and formed a germinal mass of cells between the outer and inner epithelial layers. This germinal mass formed an epithelium, some of the members of which acted as scavengers for the inner and outer layers of the host, with the result that fluid accumulated between the two parts of the germinal epithelium in connection respectively with the external and internal epithelial surfaces of the host, and thus led to the formation of a gonocœle, which, by obtaining an external opening, a cœlomostome, gave origin to the cœlom.

Again, with the longer life of the host, the setting free of the gonads no longer necessitating the destruction of the host, and also the gonads themselves requiring a longer and longer time to be fed up to maturity, the bulk and complexity of the whole organism increased and special supporting structures became a necessity. The host itself could and did provide these to a certain extent by secretions from its epithelial elements, but the intermediate supports were provided by the system of phagocytic cells utilizing the fluids of the body, at first in the shape of plasma-cells able to move from place to place, then settling down to form a connective tissue framework, and, later on, cartilage and bone.

So also were gradually evolved the whole of the endothelial structures; the lymph-cells, blood-cells, etc., all having their origin from the free cells of the body, which themselves originated in the extension of a germinal epithelium. Just as in a bee-hive the egg-cells may form the fully developed sexual animal, whether drone or queen bee, or the asexual host of workers, so in the body of the Metazoa the free cells may form either male or female germ-cells spermatozoa, or ova, or a host of workers, scavengers, repairers, food-providers, all useful to the community, all showing their common origin by their absolute independence of the nervous system.